The present invention relates to a cost saving power sharing design suitable for apparatus having dual power sources. The invention is particularly suitable for the design of electronics apparatus having one of the power source to be of limited current capability but with unlimited energy capacity. The other power source has much higher current capability but of limited energy capacity.
Traditional designs of electronics products make use of a single power supply to power the product. If a product is portable, batteries are used. If a product is stationary, a transformer circuit, switching supply or external adapter converting high voltage city power into low DC voltage is used to power the analog and digital circuits of the product. In some applications such as audio boom box and cordless phone, combination of batteries and city power can be used. Other than all these traditional power sources, limited current power supply can be obtained from some communication line, which connects the apparatus to an external device. Typical examples of these energy supplying communication lines are telephone line and RS232 serial port. In particular, the early type of USB interface provided by many computers is able to source up to 500 mA of current to the connecting equipment.
The present invention is directed to the circuit design of a low cost high efficiency power distribution circuit making use of the characteristics of the untraditional power source, such as the RS232 serial port, parallel port and USB port to support powering a product, so as to minimize the product cost and/or running cost of the traditional power supply, such as the adapter or batteries to be used.
The invented design originates from in depth study of the operation modes of an electronics apparatus. In an application embodiment, the first requirement of an useful application of the invention is that the circuitry of an electronics apparatus should be operated in at least two operation modes, an occasional high current mode and a normal low current mode. The second condition required is that an unlimited energy capacity but limited current capability power source is available during the normal operation of the apparatus. A typical example of this kind of power supply is the communication line previously mentioned. The third requirement is that the current limited power source is adequate to power the low current operation mode but inadequate to power the high current operation mode. The fourth condition required is that the apparatus is in the low current operation mode most of the time, the high current operation mode is momentary or occasional. When all these conditions are met, the power system of the apparatus can be configured in a way that the normal low current operation mode is powered by the limited current power source. The traditional power source such as batteries or city power is configured to supplement operating power during the high current modes only. In the situation of a battery powered apparatus, the battery life of the product is much longer because battery power is utilized only during the very short and less frequent high power operation durations. As a result, batteries are to be replaced less frequently and the product is more environmental friendly.
In a first embodiment, an in house paging system is designed to page employees inside an office building. Multiple low power RF transmitters are connected to the telephone line sockets of the building. The transmitters of the paging system normally stay at a low current standby mode waiting to receive a paging instruction. Normally the activation circuit and logic circuit of the transmitter unit are adequately to be powered by the energy supplied by the telephone line. When an activation signal is received, each transmitter transmits the RF paging signal. The RF transmitter requires a higher power than the current can be provided by the telephone line. In this application, the transmitting power or current of the transmitter can be obtained from a battery, AC power converting circuit or adapter connected to the city power supply. It is more user friendly for the transmitter units to be self powered or that it can be installed at any location remote from a power socket. When internal or externally connected batteries are used, in accordance to the invention, the battery power is switched to the RF circuit only when a paging signal is transmitted by the RF circuit, which happens occasionally. During the low power standby mode, the batteries are completely isolated from the application circuit; and the telephone line powers the receiving and logic circuit of the transmitter unit. Alternatively, it is more preferable for the batteries to be rechargeable and receives small charging current from the telephone line when the transmitter is idle. In this way the transmitter unit is maintenance free because the batteries servicing the higher current RF transmission mode are not to be replaced.
If a limited current communication line is not available, or the power delivered by the communication line is inadequate for the low current operation mode, the target electronics apparatus is preferably to be powered by a transformer circuit, switching power supply circuit or an adapter connected to the city power. Usually the cost of the switching supply, transformer circuit, or external adapter is proportional to the power rating of the power converting circuit. When higher power is to be delivered, more material is required to build the power converting circuit and the cost of the transformer, switching supply or adapter will be higher. If the normal operation power of the apparatus is low but the occasional power of an operation mode is very high, traditionally a high power conversion circuit design is required to meet the power requirement of the apparatus during the occasional high current operation mode. The cost of the relative expensive power conversion circuit can be reduced if a second battery or rechargeable power supply is added into the system just to supplement or power the short moment of the peak power required by the high current operation mode. A typical example of this kind of operation is a portable disk drive, R/W CD ROM or tape drive (a back up apparatus) used to backup data of a computer. The standby mode of the back up apparatus takes very small current, usually less than 30 uA. If the back up apparatus is reading or writing data, the current goes up to around 300 mA for powering the spinning motor. A momentary high power condition is defined by the moment when the motor is first started. The start up current of a motor may be as high as 800 mA, or higher. The most demanding situation of the power requirement is when the diskette or tape cassette is to be ejected by a solenoid, the solenoid used in the ejection mechanism may requires a peak current of 1.2 ampere or higher. In this application example, the power adapter is to be rated to 1.2 A in order for the unit to work properly. If a battery is provided inside the unit to supplement the momentary power required during the short moment when the motor is first started or when the solenoid is energized, the power adapter can be designed to handle only 350 mA, a limited current just enough to handle the running current of the motor. In this way size of the power adapter is shrank, cost is cheaper, less material is used and the product is more environmental friendly. Usually two AA batteries installed inside this drive can last over one or two years because the supplemental battery power is used only during the occasional start up of the inductive motor and solenoid, and that the duration of high current operation usually lasts a fraction of a second. If the data back up apparatus is connected to a USB port, the USB bus will supply 500 mA which is adequate to power the electronics circuit and normal running of the motor. In this application, with a supplemental battery, the external power adapter can be completed eliminated. It should be noted that inductive load are not limited to motor and solenoid, any other coil wound components such as coupling transformers and relay are included.
In order for the product to be more user friendly and care free, it is beneficial to have a small capacity rechargeable battery built inside the above mentioned application example. The battery is smoothly charged by the smaller size adapter or USB port during the idle time of the back up apparatus. The stored energy is then discharged to supplement the current required for starting up the motor or energizing the solenoid.
The novel features of the invention are set forth with particularly in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.